The present invention relates to a demodulator circuit for frequency-modulated signals (FM signals), and more particularly, to a demodulator circuit that is suitable to be built in a semiconductor integrated circuit.
As demodulator circuits for FM signals, a stagger-tuned frequency discriminator, a Foster-Seely frequency discriminator, and the like have been heretofore known, and these have been widely used when constructed with discrete parts because these circuits are all simple in circuit construction. However, these circuits were not suitable to be built in a semiconductor integrated circuit because of the disadvantages that they necessitated many capacitors and inductors and also that adjustment of capacitance of the capacitors and/or inductance of the inductors was necessitated.
A demodulator circuit suitable to be built in a semiconductor integrated circuit is shown in U.S. Pat. No. 3,519,944 granted to Jack Avins. In this demodulator circuit, first and second transistors are connected in a differential form, the respective bases of these first and second transistors are coupled to a grounded point via respective capacitors having a small capacity such as, for example, 10 pF, and the collector of the first transistor is connected directly to a power supply, while the collector of the second transistor is coupled through a load resistor to the power supply. In addition, the circuit comprises third and fourth transistors, the emitter-collector path of the third transistor is connected in parallel with the base-collector path of the first transistor, and the emitter and collector of the fourth transistor are connected respectively to the base of the second transistor and to the collector of the first transistor. Between the bases of the third and fourth transistor is connected a parallel resonance circuit consisting of an inductor having an inductance of 20.2 .mu.H and a capacitor having a capacitance of 62 pF, between the base of third transistor and the ground is connected a capacitor having a small capacitance of 6.8 pF, and between the base of the fourth transistor and the ground is connected a capacitor having a small capacitance of 9.0 pF. An input FM signal is applied to the base of the third transistor, and a demodulated output signal is derived from the collector of the second transistor.
Owing to the parallel resonance circuit and the two capacitors having small capacities of 6.8 pF and 9.0 pF at the respective bases of the third and fourth transistors there are produced signals that have equal amplitudes and opposite phases at a center frequency (Wo), and as the frequency deviates from the center frequency (Wo), one signal increases in amplitude while the other signal decreases PG,4 in amplitude with a phase difference of 180.degree. maintained therebetween. In this demodulator circuit, demodulation of FM signals is achieved by the first and second transistors and the two capacitors having a small capacitance of 10 pF on the basis of a similar principle to the stagger-tuned frequency discriminator.
Although the aforementioned Anvins FM demodulator circuit can be formed in a semiconductor integrated circuit because of the fact that the number of inductors is only one and every capacitor used therein has a small capacitance, the circuit has disadvantages that at least the single inductor must be provided as an external circuit, that at least five capacitors are necessitated, and if they are formed on the same semiconductor substrate, then a substrate having a considerable area is necessitated, that allowance for circuit parameters of the inductor and capacitors is narrow and thus adjustment of the circuit parameters is required, and that the number of kinds of parts is large and the circuit construction is complex.